*4.2. Prebiotics*

Since 2007, prebiotics have been defined as a "nonviable food component that confers a health benefit on the host associated with modulation of the microbiota" [64]. Prebiotics are basically classified as disaccharides or oligosaccharides and are resistant to enzymatic and chemical breakdown until they reach the colon, where they are fermented by colonic bacteria, stimulating the generation of microbial metabolic products such as short-chain fatty acids (acetate, butyrate, and propionate) [65]. Short-chain fatty acids give several benefits to the colonocytes, such as an energy source, regulation of electrolytes and water absorption, enhanced blood flow, and oxygenation [66]. Moreover, probiotics may promote host health and modulation of GI motility, reduction in visceral hypersensitivity, downregulation of low-grade mucosal immune activation, improvement of epithelial permeability, enhancement of gut–brain communication, and restoration of intestinal dysbiosis. Thus, these data provide a mechanistic rationale for a role of prebiotics in managing IBS symptoms. Indeed, several clinical studies have examined prebiotics' performance in ameliorating symptoms of functional bowel disorders. A handful of RCTs evaluating the e fficacy and safety of prebiotics in IBS [67,68] have been performed. In 1999, a small, double-blind crossover trial of oligofructose published by Hunter and colleagues [69] showed no therapeutic value in IBS patients. One year later, a randomized double-blind trial on almost 100 patients with IBS receiving either fructo-oligosaccharide or placebo for 12 weeks reported no statistically significant improvement in symptoms [70]. The first study to report a beneficial effect was a cross-over, single-blinded trial including 60 Rome II-defined IBS patients. Patients were randomized for four weeks to receive a low or high dose of trans-galacto-oligosaccharide or placebo. Both the low and high-dose arms experienced a significant improvement in stool consistency (*p* < 0.05), flatulence (*p* < 0.05), and bloating (*p* < 0.05) as well as a reduction in mean subjective global assessment (The subjective global assessment of relief was recorded at weekly intervals during the course of the study scored from 1–5. 1 = completely relieved, 2 = considerably relieved, 3 = somewhat relieved, 4 = unchanged, and 5 = worse) [71].

Interestingly, the prebiotic but not the placebo significantly enhanced fecal bifidobacteria [71]. A 12-week administration of partially hydrolyzed guar gum in a randomized, double-blind, placebo-controlled study led to a significant improvement in bloating and gasses scores with no effect on other reported IBS symptoms or quality of life scores, leading the authors to support its administration for IBS patients with an expected clinical e ffect on bloating and gasses [72]. In safety terms, all studies reported comparable rates of adverse events to the placebo arms. In conclusion, prebiotic use in IBS patients have yielded mixed to positive results, but further studies to address the combination, duration, and di fferent aspects of e ffects on IBS are still warranted.

### *4.3. Non-Absorbable Antibiotics*

Non-absorbable antibiotics, mainly rifaximin, have been shown to be safe and effective for the treatment of IBS with the diarrhea predominant type. Rifaximin, a rifamycin derivative, is a broad-spectrum, non-absorbable antibiotic which targets aerobic and anaerobic bacteria residing in the GI tract. Less than 1% of rifaximin is absorbed in the systemic circulation, making it very safe with extremely low toxicity and adverse events rates [73]. The proposed mechanisms of action of the non-absorbable antibiotics are the reduction of the amount of inhabitant GI bacteria, changes in bacterial structure, reduction of low-grade inflammation, and amelioration in gu<sup>t</sup> permeability [74]. Rifaximin is the best-studied non-absorbable antibiotic for symptoms relief in IBS. The TARGET 1 and TARGET 2 trials, both designed as double-blinded, placebo-controlled, multi-center studies, have shown good efficacy of rifaximin for IBS symptoms relief [75]. In TARGET 1 and TARGET 2, a total number of 1258 patients with mild to moderate symptoms of IBS were randomized either to receive rifaximin 550 mg three times a day for 14 days or a placebo. Relief of IBS symptoms, after one month from the end of treatment, was reported more significantly among patients in the rifaximin group compared with those in the placebo group (40.7% vs. 31.7%, *p* < 0.001), with a comparable adverse events occurrence in both groups [75]. TARGET 3, a randomized, placebo-controlled study including 2579 patients with IBS, revealed that the durability of symptoms relief among patients with IBS-D responding to a 14-day course of rifaximin was reduced by 50% after 70 days from the end of treatment [76]. With a second treatment course, the most significant benefit was the relief of stool urgency and abdominal bloating [76].

#### *4.4. Fecal Microbiota Transplantation*

Fecal microbiota transplant (FMT), also known as a stool transplant, is the process of transplantation of fecal bacteria from a healthy individual into a recipient. FMT involves the restoration of the colonic microflora by introducing healthy bacterial flora through the infusion of stool (e.g., via colonoscopy, enema, rogastric tubeor by mouth in the form of a capsule containing freeze-dried material) obtained from a healthy donor. To date, FMT has been approved for the treatment of resistant *Clostridium di*ffi*cile* infections [77,78], and in this context, it has been shown to be an e ffective therapy [79]. However, its therapeutic e ffect among IBS patients is still emerging. As reported above, in the last few years, several studies have underlined the role of dysbiosis among patients with IBS, with lower *Lactobacillus* spp. in IBS-D and increased loads of *Veillonella* spp., and genera *Coprococcus*, *Collinsella,* and *Coprobacillus* [42,80]. Other data revealed a decrease in the biodiversity of microbiota in the fecal composition of IBS-D patients [51]. Therefore, targeting the gu<sup>t</sup> microbiota composition might be a promising therapy in IBS. However, among IBS patients, FMT has shown conflicting results. A previous randomized controlled study showed beneficial e ffects of FMT on IBS symptoms. In this study, 55 patients with IBS-D or IBS-M received 50–80 g of feces mixed with 200 mL of isotonic saline and 50 mL of 85% glycerol, administered to the cecum by colonoscopy, and were compared to 28 patients who received a placebo. Patients treated with FMT showed a significant clinical response at three months compared to those in the placebo group (65% vs. 43%, *p* = 0.049) [39]. These findings were further confirmed by other recent studies showing a positive e ffect on IBS symptoms after transplant, as 70–85% and 45–60% of patients reported symptomatic relief in the first three months and six months after FMT, respectively [81,82]. On the contrary, a recent study including 52 IBS adult patients who were randomized to either active FMT or placebo capsules administered for 12 months did not show beneficial e ffects favoring FMT at three months. In fact, a significant improvement in IBS symptom scores was observed at three months favoring the placebo (*p* = 0.012), and after three months, the results obtained by an IBS quality of life questionnaire were in favor of the placebo (*p* = 0.003) [83]. Moreover, a recent meta-analysis including eight single-arm trials (SATs) and 5 RCTs did not report beneficial e ffects of FMT among patients with IBS (relative risk = 0.93, 95%, confidence interval (CI (confidence interval) 0.50–1.75, *p* = 0.83 for RCT), while in the SAT 59.5% of patients (95% CI 49.1–69.3) showed a significant improvement [84]. Given the controversies regarding the published data, FMT-based treatments for IBS are still not widely accepted among gastroenterologists, as they are

concerned about effectiveness and safety profile [85]. Therefore, the role of FMT must be further addressed by randomized, double-blind, placebo-controlled studies. Recent international consensus of stool banking for FMT have involved experts from Europe, North America, and Australia who proposed consensus guideline statements regarding several issues in stool banking, including the selection of donors and screening objectives, collection and processing of stool samples, monitoring of outcomes, ethical issues, and the evolving role of FMT in *Clostridioides difficile* infection and other diseases encountered in daily clinical practice [86]. Figure 2 demonstrates the established and evolving therapeutic options for IBS.

**Figure 2.** Treatments of irritable bowel syndrome (IBS) by targeting the gu<sup>t</sup> microbiome.
